<< K, do you honestly think theyd be stupid enough to pass up a battle like that? I bet that battle is priority no. 1 for WETA right now. Just because it wasn't covered in much detail in the book doesn't mean they won't focus on it more in the movie >>
I wouldn't necessarily bank on them covering the fight either. You'll notice that the Writer/Director (Peter Jackson, I think) tried to keep the timeline pretty linear so there wouldn't be any flashbbacks in the first movie, and if he continues that trend in the next one then we won't hear much about the fight at all when Gandalf returns because it doesn't look like he favors flashbacks and the only way to avoid a retelling of the fight in the form of a flashback would have been to show the continuation of it while the rest of the group continued the journey.
Now in this case you might say that he wants to keep the non-readers in suspense until the middle of the second movie about it, and that very well might be the case, but he chose to basically eliminate the under-riding theme of the first part of the Fellowship which is what happened to Gandalf? In the book, the reader was forced to join Frodo in wondering that through the entire journey from the Shire to Rivendell and then Gandalf's activities were retold at Rivendell. Same thing happened in the Two Towers when Gandalf disappeared for the whole first half and then reappears in the second half to tell what happened to him.
I suppose they could show the fight right at the beginning of the second film to catch you up as to what happened to Gandalf because you would be finding out about it in an hour or so anyway, but who knows for sure?
Complete conjecture on my part obviously.